Method and electrolyte for thick,brilliant plating of palladium



United States Patent US. Cl. 204-43 3 Claims ABSTRACT OF THE DISCLOSUREPalladium is electroplated to give a thick, bright deposit. The aqueouselectrolyte comprises ammonia; a palladium salt and an amount ofpyridine, pyridine carboxylic acid, or pyridine carboxylic acid aminesufiicient to give a bright deposit. A lead salt may also be present insolution to assist in providing a bright deposit.

This invention relates to a plating bath composition for electroplatingpalladium on metal surfaces.

At present white colored plating on the component parts of weak electricapparatus, jewelry, etc., is usually of silver or nickel. Platingprocedures for plating silver and nickel were developed many years agoand nowadays they are popularly adopted for industrial uses. The platedsurfaces resulting from utilizing these methods of plating resulted insome drawbacks with respect to corrosion resistance, anti-weatheringproperty, wear resistance, electric properties, color, etc. Manymeasures had to be taken to counter them. In spite of suchinconveniences silver plating and nickel plating are widely adoptedbecause there are no alternatives.

Recently the need for higher quality electroplated apparatus and jewelryhas developed. This high quality cannot be obtained by conventionalplating procedures. Therefore the need for better plating procedures isgetting more and more urgent. We have succeeded in electroplatingpalladium to eliminate the drawbacks of the surfaces plated by theconventional methods. Palladium plating is superior to the conventionalones in respects of corrosion resistance, anti-weathering property andwear resistance. So it can satisfy the practical necessity of todaysrequirements.

Although palladium plating is inferior to silver plating in electricproperties, e.g. it has a large electric resistance, superior incorrosion resistance and anti-weathering property, and it proves betterquality under severe circumstances or after long term usage. Moreover itgives a beautiful color which is durable and different from that ofsilver or nickel.

There are several kinds of baths introduced since l930s, though theywere seldom used practically. Those baths are largely classifiedaccording to the composition as follows:

(1) Those which contain ammonium solution of Pd(NH (NO as a principalingredient: Ex. Keitel, Trans. Electrochem. Soc., 59, 275 (1931).

(2) Those which contain aqueous solution of K Pd (N0 as a principalingredient: Ex. Wise, U.S.P. 1,970,950 (1934); Raper, U.S.P. 1,993,627(1935).

(3) Those which contain aqueous solution of K PdCl as a principalingredient: Ex. Wise U.S.P. 2,335,821 (1943); 2,457,021 (1948).

(4) Those which contain aqueous solution of Pd(NH (NO as a principalingredient: Ex. Laister, Metel Ind, 85, 428, (1944); Philpott, PlatinunMetals Rev., 4, 12, 1960) We thoroughly investigated and experimented onabove listed baths. As a result it was found that none of those bathscould giv satisfactory brilliant plating, i.e. with those baths we couldnot get brilliant surfaces when the thickness of plating was more than0.5 1O' cm. As the thickness of the plate exceeded this value, platinggradually became dull greyish black. Such a color is very undesirable asto good appearance. Generally, brilliance is indispensable when we platea merchandise to raise its value. On the other hand sufiicientproperties of corrosion resistance, anti-weathering, wearing resistance,etc. usually cannot be obtained on the palladium plated surfaces whenthe thickness of plating layer is only 0.5 X 10 cm., the upper limit ofthickness which gives brilliance. Here we fall into a dilemma. Buffingmay be considered to improve brilliance, but it is not only expensivebut also impossible on many weak electric apparatus parts and jewelry.

Thus we had to conclude that the plating baths commonly employed werenot practical. So we started developing a new bath suitable for thickbrilliant plating. Accordingly the object of the present invention is toprovide a bath for electroplating palladium, which can give a platinglayer of practically sufficient thickness and brilliance withoutsacrificing the easiness of operation and the beneficial properties ofresultant plating layers.

(a) We composed many baths, each having ammonia solution of one of thevarious palladium salts as a principal ingredient, to make experimentson the effects of plating. During these experiments we found that wecould improve brilliance by adding pyridine. The plated surface hadbrilliance until the plating grew as thick as about 3X 10* cm. At 5 X10" cm. the surface showed slight milkwhite blur, which could easily bemirror polished by buffing. Conventional baths cannot give such plating.The resulting plating of this bath fully satisfies the present practicalrequirements. We also found that brilliance was maintained until theplating grew as thick as at least 10 10 em, if EDTA-Pb (chelate compoundmade from ethylenediaminetetraacetic acid and lead iron) was added tothis bath.

This knowledge constitutes the essential part of this invention.Accordingly the plating solution of this invention contains one ofvarious palladium salts dissolved in ammonium hydroxide, and alsoEDTA-Pb if required. Aforesaid objects of this invention are achievedwhen the bath is controlled so that the concentration of palladium saltas metal is 1-50 g./ 1., concentration of pyridine more than 0.01mol./l., concentration of EDTA-Pb as Pb 0.05- 1 g./l., alkalinity of thesolution more than pH 9.5, temperature 1050 C., and cathode currentdensity 0.1-l0 A./dm.

(b) Palladium concentration of this bath is between 1 g./l. and 50 g./l.considering the purpose of lating and economical requirements, providedthat low cathode current density be used for low palladiumconcentration. Palladium concentration of more than 15 g./l. is usuallynecessary for cathode current density of 1 A./dm.

Dichlorodiaminepalladium Pd(NH Cl is the most suitable source ofpalladium, because this material is easily prepared and purified, andalso because this material is rapidly dissolved in ammonium hydroxide.As palladium salts are dissolved in ammonium hydroxide, palladium existsin the solution as an ammonium complex salt whatever the kind oforiginal salt might have been.

Ammonium solution of palladium salt is defined as solution in whichpalladium exists dissolved as ammonium complex salt.

(c) Pyridine is an important ingredient of this bath, but it isinconvenient to use this material because of its offensive odor. So wetried to use derivatives of pyridine. Pyridine 3-carboxylic-acid andpyridine 4-carboxylic-acid brought satisfactory results. Position of thecarboxyl radical in the pyridine nucleus has no effect on plating.Nicotinamide UCONH:

and N-diethyl-3-pyridine-carboxylic-acid-amide CON also produced goodresults.

These materials are oxidized by the anode to change into correspondingpyridine-carboxylic-acids during electroplating process. Then it isunderstood that the effects provided by these materials are broughtbecause of their pyridine nucleus and that the sort and the position ofsubstituent have no bearing in the process. Of course such derivativesare not suitable that have substituents which are not chemically stablein electrolyzing process. For example4-pyridine-carboxylic-acid-hydrazide OoNHNHr did not bring good result.The lowest efiective concentration of these materials is 0.01 mol./l.and there are no undesirable results even if the concentration is raisedhigher than this, for example 2 mol./l.

Anyway, plated surfaces produced from the baths which contain pyridine,pyridine-carboxylic-acid, or pyridine-carboxylic-acid-amide areevidently superior to those from conventional baths.

(d) When brilliant plating thicker than 5 X cm. is required, goodresults are obtained by adding EDTA-Pb to the bath. Concentration ofEDTA-Pb in the bath should strictly be maintained within 0.05-1 g./l. asPb. Otherwise EDTA-Pb has no etfect or even causes bad results.

As EDTA-Pb is added the plating layer becomes an alby which contains1-2% of lead, and accordingly amount of lead in the bath graduallydecreases. Then lead must be replenished as EDTA-Pb.

Although it is not explained how EDTA-Pb operates to give brilliance,the concentration of lead in the bath is supposed to have significantinfluence. We made experiments on the baths to which ions of leadnitrate, lead chloride, etc., or weak organic complex salt such as leadacetate, lead citrate, etc., was added instead of EDTA-Pb. Such bathsalso gave brilliant plating. But lead concentration in the baths had tobe kept very low, i.e., 1-5 mg./l., and as a natural result they did notbring constant effects. So those baths cannot be .expected to bepractically useful.

On the other hand the baths to which strong chelate compounds such asCyDTA-Pb(l-2-cyclohexylenedinitrilo-tetraacetic lead) and DTPA-Pb(diethylenetriaminepentaacetic lead) were added constantly gave the samebrilliant plating as we got from the bath which contains EDTA-Pb whenthe lead concentration was within 0.05-1 g./l.

Summarizing the above, the primary cause of brilliance is the presenceof lead in the bath. Concentration of lead should be kept within adefinite range determined by strength of complicating agent.

(e) When the bath of the present invention is utilized, there isinterrelation between palladium concentration, temperature of the bathand cathode current density. Cathode current density should be loweredas palladium concentration decreases and bath temperature becomes low.The bath usually gives a good result at palladium concentration of morethan 15 g./l., temperature of 30- 40 C. and cathode current density of lA./dm.

The plating bath is made alkaline by means of the addition of ammonia.Generally a pH of above 9.5 is desirable.

Plating is nearly mirror-brilliant at the thickness of 3 l0 cm., and at5 l0 cm. presents slight milk white blur which can easily be buffed ofi.Addition of EDTA-Pb is necessary for brilliant plating thicker than 5 10cm. It is able to give brilliance to the plating at lease as thick as 1010' cm.

Current efliciency is always -95 and troubles which may be caused byhydrogen absorbed, i.e., embrittlement caused by hydrogen, do not ariseon the plated surface.

Anode should be metal palladium plate or black lead electrode. Aspalladium concentration in the bath decreases, palladium salt such asdichlorodiaminepalladium Pd(NH Cl should be added.

Proper pH of the bath should be maintained by adding concentratedammonium hydroxide. Various inorganic or organic salts may be added tothe bath to raise electric conductivity.

Preparatory treatment on the metal to be plated is processed in the sameway as usual. There are no restriction as to the kind of the metals onwhich palladium is lated.

Execution 1 Dichlorodiaminepalladium g./l 40 Pyridine g l 5 Ammoniumhydroxide (the amount which makes pH of the bath 10) percent 25 Cathodecurrent density A./dm. 1 Temperature C 35 Without agitation.

Under these conditions plating was brilliant until it grew as thick as 310- cm. At the thickness of 5 X10 cm. slight milk white blur appeared,but it was easily butfed off. There was no cracking, and plating welladhered to the base. Current efficiency was Without agitation.

Same results as Execution 1 were obtained. The bath which containedpyridine 4-carboxylic-acid instead of pyridine 3-carboxy1ic-acid alsoresulted same plating.

Execution 4 Dichlorodiaminepalladium g./l 40 Nicotinamide g/l 5 Ammoniumhydroxide (the amount which makes pH of the bath perccnt Cathode currentdensity A./dm. 1 Temperature C Without agitation.

Same results as Execution 1 were obtained. The same results were alsoobtained when more nicotinamide was used to make its concentration 150-g./l.

Execution 5 Dichlorodiaminepalladium g./l Nicotinamide g/l 5 EDTA (Nasalt) g./l Ammonium hydroxide (the amount which makes pH of the bath 10)"percent" 25 EDTA-Pb (as Pb) mg 400 Cathode current density A./dm. 1Temperature C 35 Without agitation.

With above bath composition and plating conditions, plating wasperformed until layer grew as thick as 15X l0- cm. Mirror-likebrilliance resulted and no cracking took place. Plating layer welladhered to the base. Current efiiciency was Same results were obtainedwhen potassium chloride was added to the bath in the proportion of 50g./l. to raise electric conductivity.

We claim:

1. A method for obtaining a thick, brilliant palladium platingcomprising electroplating a metal in a diphragrnless electroplating cellwhich contains an aqueous ammoniacal solution maintained at a pH ofabove 9.5, comprising ammonia, a palladium salt in an amount sufficientto supply a concentration of palladium metal of from 1 to 50 grams perliter, and a brightening agent for brightening the palladium platingselected from the group consisting of pyridine, pyridine carboxylicacid, and pyridine carboxylic acid amides, said agent being present inthe bath in an amount of from about 0.01 to 2 mols per liter,maintaining the current density in said bath during said lating, at from0.5 to 1 A./dm. and maintaining the bath at a temperature of about 35 C.during said plating.

2. The method in accordance with claim 1 wherein said bath contains alead complex consisting of lead chelated with an organic chelatingagent, said complex being present to provide lead in an amount of fromabout 0.05 gram to 1 gram per liter.

3. An electroplating bath comprising an aqueous ammonaical solutioncontaining a palladium salt, said salt being present in an amountsufficient to provide palladium metal in a concentration of from about 1to 50 grams per liter, a brightening agent selected from the groupconsisting of pyridine, pyridine carboxylic acid, and pyridinecarboxylic acid amide, said agent being present in the bath in an amountof about 0.01 to 2. mols per liter, and a lead complex consisting oflead chelated with an organic chelating agent, said complex beingpresent to provide lead in an amount of from about 0.05 gram to 1 gramper liter.

References Cited UNITED STATES PATENTS 1,921,941 8/1933 POWCll et al.204*47 1,981,715 11/1934 Atkinson 204-43 2,335,821 11/1943 Wise et al.20447 2,452,308 10/1948 Lamhros 204-47 JOHN H. MACK, Primary Examiner G.L. KAPLAN, Assistant Examiner US. Cl. X.R.

